The present disclosure relates to systems and methods for making direct reduced iron. Direct reduced iron (DRI) is a commercial product of more than 80% metallic iron, and, typically, more than 90% metallic iron, widely used as a source material for making steel. The remainder of the DRI product is gangue, which is high in silica. The conventional techniques for making steel involve the use of an electric arc furnace (EAF) or a basic oxygen furnace (BOF). DRI is typically higher in iron units than taconite pellets and other sources of iron, and can be used as a partial substitute for scrap in production of steel by EAF.
DRI may be formed from beneficiated iron ore, such as taconite pellets. For example, taconite has been mined and crushed, and the iron containing portions magnetically separated from the non-magnetic portions to form a beneficiated product substantially higher in iron content than mined taconite. The beneficiated iron ore portion may be formed into pellets by pelletizing, and heated in a linear hearth furnace (LHF) in the presence of a reducing agent (e.g., carbonaceous material) to a temperature below the melting point of iron using natural gas, propane, or coal to promote the reduction of iron ore to form DRI.
In the process to make DRI, the beneficiated and pelletized iron oxide containing material is moved through a furnace mixed sometimes with a reducing agent, such as coal, coke, or another form of carbonaceous material. A desulfurizing agent, such as limestone or dolomite, may also be added. The iron oxide material reacts chemically in the reducing zone of the furnace to partially reduce the iron oxide to form DRI.
DRI is difficult to transport because it is highly reactive with oxygen in air and moisture. The DRI, known as sponge iron, has a high porosity with many voids making it porous in nature. With the high porosity of DRI, it has low compressive strength. When the DRI is stored, for example, in the hold of a ship or barge during transportation, some of the pellets have been prone to break apart under the weight of pellets above them, further promoting the likelihood of the DRI reacting with oxygen and moisture around it. Additionally, the rough surface characteristics of the DRI pellets produce particulate matter and other fines having a high surface area, which also promoted the likelihood of the DRI reacting with the oxygen and moisture around it. Such particulate matter and fines typically are produced throughout the transportation and storage of the DRI, making it difficult to transport DRI over long distances and to store DRI for long periods.
DRI is usually made, transported, and stored in the form of DRI pellets, accompanied by DRI dust and other DRI fines. The porous, low internal strength, and flakey nature of such DRI compacts work to increase the surface area of the compact that is exposed to an oxidizing atmosphere and moisture, resulting in substantial and rapid oxidation (rusting). The amount of DRI fines is thus increased during transportation and storage of the DRI product before delivery to the steelmaking furnace. The reactions that occur during DRI oxidation produce heat and hydrogen, making DRI susceptible to overheating and combustion. The temperature in containers storing DRI, in which air is free to circulate, can reach 1200° F. Such combustion may cause fires in the holds of ships and barges during transportation of DRI and even in the clam shell buckets of cranes when unloading DRI. These circumstances have substantially increased the cost of DRI product delivered to a steel plant because of the losses in handling and hazards encountered during transportation and storage. Due to the difficulties and risks associated with transporting DRI product, production of DRI has, with a few exceptions, been generally located near the steelmaking facilities and near the time of use in steelmaking, rather than in more economical locations and times.
Consequently, various techniques have been used in the past to passivate DRI to reduce the risks associated with its pyrophoric properties. Examples of such passivation chemistry used in the past include aqueous solutions of a water soluble alkali metal silicate coating, organic amine vapors or aqueous solution of organic amines coating, petroleum wax with a solid polymer of an olefin having 2 to 4 carbon atoms coatings, water soluble stearates and like water repellent additive coating, hydrated calcined limestone (lime dust) coating, polymerizing aliphatic 1-olefin of less than 6 carbon atoms with catalytic material coating, ferrific chromium containing alloy coating, naphthenic petroleum/glycerol monoester coating and heated paraffin wax coating.
Natural triglycerides such as soybean oil, sunflower oil, coconut oil, cottonseed oil, and castor oil have also been proposed as a nonaqueous foam for use as a dust suppressant with iron ore pellets. See WO 2006/010721. These dust suppressants are typically applied in the presence of a surfactant such HCF-740, a mixture of florosurfactants and hydrocarbon solvents, HCF-730, a nonionic mixture of silane surfactants, HCF-720, a nonionic mixture of silane surfactants and fluorosurfactants, or HCF-710 a nonionic mixture of silane surfactants and sulfonic acids. Id at 4. Such surfactants have been proposed to be used in amounts of 0.2 to 5% by weight per weight of DRI dust or fines. While iron ores are generally relatively stable oxides, DRI pellets and DRI fines generally have high porosity and high reactive surfaces, and known as sponge iron. Accordingly, DRI pellets and DRI fines in the presence of these natural triglycerides tend to break down the triglycerides producing high levels of CO, which is highly undesirable with transportation of DRI.
Moreover, such treatment techniques generally have involved separation of the dust and other fine particles from the DRI pellets, and discarding the separated fines as waste. This involved reduction in yield and loss of iron units because the dust and other fines were fugitive and not recovered.
Despite various attempts, there still remains a need for an economic and efficient way of passivating DRI pellets so they can be safely transported over long distances in bulk and stored in bulk. A strong, stable and pyrophobic product would enable the safe transport and storage of DRI, substantially increasing its usefulness and effectiveness in steelmaking. A need has existed for both reclaiming and inhibiting activation of the iron units of DRI fines to improve the efficiency of the use of DRI pellets in steelmaking, while reducing the risks in transportation and handling of DRI pellets.
Disclosed is a method of reclaiming and inhibiting activation of DRI fines comprising the steps of: (a) forming a moving stream containing DRI pellets and DRI fines; (b) coating DRI pellets and DRI fines with a coating material comprising a mixture of alkanes in the C15 to C40 range to form a coating on the DRI pellets and DRI fines and cause DRI fines to adhere together and to the DRI pellets to form a plurality of agglomerates; and (c) moving the agglomerates of coated DRI pellets and coated DRI fines to a facility for use in making steel.
The coating material may comprise a mixture of alkanes in the C18 to C35 range from a non-vegetable source. In some embodiments, the coating material may be at room temperature. In other embodiments, the coating material may be heated before application.
The coating material may be mineral oil. Mineral oil prevents the absorption of moisture from the atmosphere. Mineral oil may have a viscosity of 70 SUS at 100° F./5=cSt at 40° C. The viscosity of a fluid is a measure of its resistance to gradual deformation by shear stress or tensile stress. A fluid with a very high viscosity will appear to be a solid in the short term. A liquid whose viscosity is less than that of water is sometimes known as a mobile liquid, while a substance with a viscosity substantially greater than water is called a viscous liquid.
The coating material may also include paraffinic oils. Paraffinic oils are based chemically on n-alkanes. Paraffinic oils are straight chain or branched aliphatic hydrocarbons belonging to the series with the general formula CnH2n+2. Paraffinic oils are saturated with respect to hydrogen. A typical paraffinic oil molecule with 25 carbon and 52 hydrogen atoms has a molecular weight of 352. Very high molecular weight paraffins are solid waxes, also dissolved in small amounts of mineral oils. Paraffins are relatively unreactive and thus have better oxidation stability compared to naphthenes. Thus, they can be used longer at higher temperatures.
Irrespective of the composition of the particular coating material used in the present method, the method may be practiced by coating DRI pellets and DRI fines to form DRI agglomerates. The method may include forming a moving stream containing DRI pellets and DRI fines. The moving stream of DRI pellets and DRI fines may be a falling stream when the step of coating occurs. The coating material may be delivered in different droplet sizes usually at the same time to facilitate efficient coverage of the DRI fines and to facilitate coverage of the DRI pellets with the coating material. The coating material may be applied at a rate between 0.2 and 2.0 gallons per ton of DRI coated, or between 0.4 and 1.0 gallon per ton of DRI coated. In one embodiment, the coating material may be a liquid or semi-liquid applied at a rate less than 0.2% by weight per ton of pellets and fines coated. In another embodiment, the coating material may be a liquid or semi-liquid applied at a rate between 0.005 and 0.45% by weight per ton of pellets and fines coated.
Alternatively, disclosed is a method of reclaiming and inhibiting activation of DRI fines comprising the steps of (a) coating with a coating material a pile of DRI pellets and DRI fines while DRI pellets and DRI fines are added to or removed from the pile, said coating material comprising an alkane mixture in the C15 to C40 range, causing DRI fines to adhere together and optionally to the DRI pellets to form a plurality of DRI agglomerates; and (b) moving the DRI agglomerates formed from coated DRI pellets and coated DRI fines to a facility for use in making steel.
Again, the coating material may comprise of an alkane mixture in the C18 to C53 range. The coating material may be heated before application to form a liquid or semi-liquid before or during the step of coating, and may be with different droplet sizes to facilitate coverage of both DRI pellets and DRI fines before or during the step of coating. The coating material may be applied at a rate less than 0.2% or may be applied at a rate between 0.005 and 0.45% by weight per ton of DRI pellets and DRI fines coated. In any case, the coating material may be applied at a rate less than 0.2% of DRI coated or may be applied at a rate between 0.005 and 0.45% by weight per ton of pellets and fines coated. The coating material may be applied at a rate between 0.2 and 2.0 gallons per ton of coated DRI coated, or between 0.4 and 1.0 gallon per ton of DRI coated.
In yet another alternative, disclosed is a method of reclaiming and inhibiting activation of DRI fines comprising the steps of: (a) coating DRI fines with a coating material comprising an alkane mixture in the C15 to C40 range to form a coating on the DRI fines and cause DRI fines to adhere together and optionally to DRI pellets to form a plurality of agglomerates; and (b) moving the agglomerates containing coated DRI pellets to a facility for use in making steel.
In an embodiment, the present method may be practiced by collecting the DRI fines before the step of coating, and the collected DRI fines may be coated and form DRI agglomerates. The DRI may comprise more than 90% metallic iron. The coated DRI agglomerates may be directly used in steelmaking, or, the coated DRI agglomerates formed from the DRI fines may be added to uncoated DRI pellets and DRI fines, or DRI fines, to facilitate coating of the uncoated DRI pellets and/or DRI fines with the coating material comprising an alkane mixture, and cause the DRI fines to adhere together and optionally to DRI pellets to form a plurality of additional DRI agglomerates. This embodiment reduces the amount of coating material that is used in efficiently practicing the present method of reclaiming and inhibiting activation of DRI fines.